1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method and system for facilitating the timing of base stations in an asynchronous Code-Division Multiple Access (CDMA) mobile communications system.
2. Description of Related Art
Direct-Sequence CDMA (DS-CDMA) mobile communications systems can be either inter-cell synchronous or inter-cell asynchronous systems. In other words, the base stations (BSs) in an inter-cell synchronous system are accurately synchronized with one another, and the BSs in an inter-cell asynchronous system are not. More specifically, asynchronous BSs do not share a common time reference, and their transmissions, therefore, have arbitrary, not predetermined timing relative to each other. An example of an inter-cell synchronous system is the North American IS-95 system. Examples of inter-cell asynchronous systems are the Wideband CDMA (WCDMA) systems proposed in the CODIT, ETSI SMG2 Group Alpha, and ARIB technical specifications.
The main disadvantage of inter-cell synchronous systems is that the BSs have to be very accurately synchronized (down to the xcexcs level). This high level of accuracy is typically provided through the use of highly accurate time references co-located with the BSs, such as Global Positioning System (GPS) receivers. However, because of the line-of-sight nature of satellite signal propagation, the use of such co-located references are likely not feasible for BSs located underground, in buildings or tunnels. Another related disadvantage is that the GPS system is controlled by a government agency. Consequently, the use of GPS receivers for BS network synchronization may be undesirable in some national regions. These disadvantages are the main reasons why inter-cell asynchronous systems are now being considered.
For inter-cell asynchronous systems to work properly, there are two crucial functional issues that need to be addressed: (1) Soft Handovers (SOHOs); and (2) Cell-Searches. In a state of SOHO, a mobile station (MS) is in communication with more than one BS at the same time. To facilitate the SOHOS, the MS constantly scans for other BSs in the vicinity. The MS can thereby monitor the received signal quality from the multiple BSs and determine the time delay of the BSs. For a SOHO to occur, the MS being handed over has to be able to receive the xe2x80x9ctargetxe2x80x9d BS""s signal at approximately the same time as the xe2x80x9csourcexe2x80x9d BS""s signal, in order to minimize buffering requirements (i.e., a smaller time difference between BS signals requires less buffer area than larger time differences). Also, the target BS has to be able to find the MS""s signal without an unreasonable expenditure of processing resources.
These SOHO issues are resolved for asynchronous systems by a xe2x80x9cper-callxe2x80x9d synchronization technique, which is disclosed in xe2x80x9cA Design Study for a CDMA-Based Third-Generation Mobile Radio System,xe2x80x9d by A. Baier et al., IEEE JSAC, Vol. 12, pp. 733-743, May 1994. Using this technique, the MS involved in the SOHO calculates and reports to the network the time difference between the target BS and source BS. The network notifies the target BS via the Base Station Controller (BSC) or Radio Network Controller (RNC) about the time difference. The target BS can then adjust its receive and transmit timing for the signal intended for the MS involved, to compensate for the difference.
A similar known SOHO technique is used in which the MS reports the timing difference between the target BS""s transmission and its own transmission, rather than the difference between the target BS""s transmission and the source BS""s transmission. However, since the MS""s transmit/receive timing relationship is always fixed, the two above-described SOHO techniques are essentially equivalent. These techniques are referred to as mobile assisted handover (MAHO). In other words, the MS assists the target BS in compensating for the difference in timing between the target BS and source BS.
A cell-search generally refers to a procedure whereby an MS accomplishes chip-, slot- and frame-synchronization with a BS, and detects the BS""s downlink scrambling code. This procedure is used both during power on (initial synchronization) and continuously thereafter during the idle or active modes while the MS is searching for SOHO candidate BSs. In a synchronous system, the cell-search can be performed efficiently (i.e., with a relatively low level of complexity) because the same scrambling code can be used by all BSs. As such, the MS can perform the complete search for BSs using only a single matched filter (or a similar functionality). However, this same technique cannot be readily used in an asynchronous system because of the different scrambling codes used by the different BSs. Consequently, a need has arisen for a low-complexity, rapid cell-search procedure for asynchronous CDMA systems.
A rapid, multi-step cell-search procedure for asynchronous CDMA systems has been proposed, whereby each BS transmits one unmodulated symbol. This transmitted symbol is spread by a globally-known short code, without a scrambling code, in each slot of each frame. In one such proposal, this symbol is denoted as a xe2x80x9cPerch 1 Long Code Masked Symbol (LCMS)xe2x80x9d. In a second proposal, this symbol is denoted as a xe2x80x9cPrimary Synchronization Channelxe2x80x9d or Primary (SCH). With the proposed multi-step procedure, an MS can thus find the chip- and slot-timing of a BS, using a single matched filter which is matched to the Primary SCH. Subsequently, the MS still has to find the BS""s frame-timing and downlink scrambling code (which spans one frame in the proposed multi-step procedure). The MS can find the BS""s frame-timing by detecting a second regularly transmitted symbol, which is denoted as a xe2x80x9cPerch 2 LCMSxe2x80x9d or xe2x80x9cSecondary SCHxe2x80x9d.
This second symbol is transmitted in parallel with the first symbol, but the second symbol is spread by a second short code (again without a scrambling code). The second symbol may also have a unique repetitive modulation pattern per frame, and by detecting this pattern, the MS can determine the BS""s frame-timing. The spreading code used for the second symbol indicates to the MS which group of possible scrambling codes an actually-used scrambling code belongs to. The MS can then find the scrambling code used, by correlating with the scrambling codes belonging to the indicated group, at the above-identified frame-timing (or at different possible frame-timings). However, a problem with the proposed multi-step procedure is that the level of complexity of the cell-search is still relatively high, especially in the case of a SOHO candidate search (which the MS has to perform on a regular basis).
Another problem with inter-cell asynchronous systems is that the timing difference between BSs makes it difficult to determine the position of the MSs. Mobile communications systems capable of determining the position of MSs in the system are becoming increasingly desirable. Currently, mobile positioning is generally performed by the use of external systems, such as a GPS system. Preferably, however, mobile positioning would be performed by the cellular system itself without the need for such external systems. To perform such cellular positioning, a method is needed to accurately determine the absolute or relative distances between an MS and each of several different BSs. The distances can be calculated using propagation time, time of arrival (TO), or time difference of arrival (TDOA) measurements on the signals transmitted between the MSs and each of several different BSs. Once these measurements are available, a number of algorithms exist to calculate the geographical position of the MS. For example, according to the TOA method, the distance from an MS to each of the BSs is obtained using TOA measurements. Each of these distances can be conceptualized as the radius of a circle with the respective BS in the center. In other words, the TOA measurement can be used to determine the radial distance of the MS from a particular BS, but the direction cannot be determined based on a single TOA measurement; thus, the MS might lie anywhere on the circle defined by the calculated radius. By determining the intersection of the circles associated with each of several different BSs, however, the position of the MS can be determined. The TDOA method, on the other hand, uses the difference in TOA between two BSs to determine a TDOA between those two BSs. The position of the MS can then be estimated to be along a curve, namely a hyperbola, in accordance with the TDOA calculation. By using three or more BSs, more than one such curve can be obtained. The intersection of these curves gives the approximate position of the MS.
In the simplest mobile positioning technique, a SOHO is made to a number of BSs. During each of these handovers, the propagation time between each BS and the MS can be measured. The location of the MS can then be determined by triangulating the position of the mobile. This positioning method is the simplest to implement because it involves very little change in the mobile radio design. In addition, the BSs do not need an absolute time reference; i.e., this method may be used in an asynchronous cellular system. However, because of the geographical separation between BSs, handover to two other geographically located BSs is only possible in a small number of cases. In other words, when the MS is in close proximity to one BS, a SOHO with other BSs will often not be possible. This is because the xe2x80x9chearabilityxe2x80x9d of signals between the MS and multiple BSs will normally be unsatisfactory.
Another possible solution is to use an antenna array at the BS. When the BS has an antenna array, the position of the MS can be calculated by estimating the direction from which uplink signals are propagating and by measuring the round-trip delay of the communications signal. In this method, the MS only needs to be in communication with one BS to calculate the position. However, widespread use of antenna arrays for positioning purposes is expensive. Furthermore, the effects of multipath propagation characteristics of the uplink and downlink signals often make an antenna array undesirable, particularly in cities, where signals frequently reflect off buildings and other structures.
As mentioned above, it is also possible that a GPS can be incorporated into the mobile without using an extra radio receiver. This method, however, requires excessive computational and receiver complexity in the MS.
Another solution is to measure the propagation time, TOA, or TDOA of signals transmitted by the BSs to the MS or by the MS to the BSs. For example, a downlink solution can be used wherein, in the case of CDMA, the MS measures the TOA of pilot channel data that is transmitted by several different BSs. Alternatively, an uplink solution can be used wherein several BSs each measure the TOA of a signal transmitted by the mobile to the multiple BSs. However, both of these methods require an absolute or accurate relative time reference in, or synchronization of, the BSs. Therefore, both downlink and uplink solutions normally require extra hardware (e.g., a GPS receiver located in the BSs to obtain timing of the BSs) in an asynchronous network.
A system and method are needed for reducing the complexity of and the processing resources used during the cell search and mobile positioning processes in asynchronous networks. In particular, it would be advantageous to utilize as much a priori search information as possible to help reduce the level of complexity and increase the search rate for cell-searches and to enable simplified mobile positioning solutions. As described in detail below, the present invention successfully resolves the above-described problems.
A method and system are provided for facilitating the timing of base stations in asynchronous CDMA mobile communications systems, whereby a source BSC (or RNC) sends to an MS (e.g., in a neighbor cell list message) estimates of the Relative Time Difference (RTD) between the source BS and each of the BSs on the neighboring cell list. For SOHO purposes, a plurality of MSs can report to the network the estimated RTDs along with signal quality information for the neighboring BSs. Each BS can maintain an RTD estimate table, which can be updated continuously from the RTD reports received from the MSs. Subsequently, the BSs can send entries from this RTD estimate table to the MS in the neighboring cell list message, along with corresponding scrambling codes. Using this novel technique, the BSs have known relative timing differences. Consequently, when the MS initiates a cell-search for a potential target BS, the MS already has an estimate of the timing of that BS as compared to its source BS. As such, the resulting cell-search procedure used in an asynchronous CDMA system has a lower level of complexity and thus can be accomplished much quicker than with prior procedures.
In another aspect of the invention, the accuracy of the estimated RTD""s can be greatly improved by accounting for propagation delays between the MS and the BSs that are used to estimate the RTD. These improved RTDs can be used to further improve timing estimates for performing cell-searches. The improved RTDs can also be used to calculate the position of MSs in the mobile communications system. Once highly accurate RTDs are known, distances between an MS and several BSs can easily be determined using the propagation times, TOAs, or TDOAs of signals traveling between the MS and the several BSs.
An important technical advantage of the present invention is that neighboring BSs in an asynchronous CDMA mobile communications system have known relative timing differences.
Another important technical advantage of the present invention is that the hardware and software complexity of MSs in an asynchronous CDMA mobile communications system is reduced.
Yet another important technical advantage of the present invention is that the overall level of complexity of the cell-search procedure in an asynchronous CDMA mobile communications system is significantly reduced.
Still another important technical advantage of the present invention is that the speed of the cell-searches performed in asynchronous CDMA mobile communications systems is significantly increased as compared to prior procedures.
Another important technical advantage of the present invention is that mobile positioning can be determined in an asynchronous mobile communications system by performing simple calculations on easily obtainable data and without the need for an external system.